The present invention relates to a gearbox with hydrostatic motors particularly for earth-movers.
Currently earth-movers often use hydrostatic drive units, which have a total conversion ratio, or c.r..sub.tot, defined in the following expression: c.r..sub.tot =(max. travel speed)/(maximum working speed under traction), which is limited to a ratio of approximately nine to one, using three times the adjustment of the hydrostatic pump and the hydrostatic motor as degrees of freedom of the above expression.
In order to improve, i.e. increase, the total conversion ratio c.r..sub.tot, gearboxes with simple clutches have been interposed between the hydrostatic motor used and the axle and, although on one hand they solve the problem of the total conversion ratio, which is provided in this case by the following expression: EQU c.r..sub.tot =c.r..sub.pump *c.r..sub.motor *c.r..sub.gearbox
improving it considerably, the earth-mover is forced to stop during gear shifting.
Due to this problem, some earth-movers have a range for working speeds, comprised for example between 2 and 18 km/h, and another range for travel speeds, i.e. 4.25 to 38.2 km/h.
An attempt has been made to obviate this last problem by using a transmission with two or more gears, but various studies have found that this last solution is often unacceptable due to the limited quality of gear shifting, i.e. due to power losses with consequent slippages and shocks. These further problems are caused first of all by the considerable gear intervals required, which are comprised within the range between 2.15 and 3.15, but most of all by the inertia that the hydrostatic system opposes to the change in rate of travel.
By using more gears with an equal total conversion ratio, it is possible to reduce the adjustment of the hydrostatic pump and motor; this permits either the use of smaller and less expensive components or alternatively permits the use of higher power with standard components. This has allowed the creation of power-shifts transmission with three or four gears which are extremely complicated and cost.
Hydrostatic motors have recently appeared on the market which allow adjustments down to practically zero displacement, thus allowing almost a doubling of the adjustment of the motor, which allows to practically double the total conversion ratio, raising it up to approximately 18, by using a pair of constantly engaged twin motors and by setting one to zero at high speeds or by setting to zero and disconnecting one of the two when it has reached its maximum speed and leaving the other motor to continue on its own, and finally allowing to change gear with a normal synchromesh unit when the motor is in the zero-displacement condition and all the power flows onto the other motor.
However, the greatest problem which now arises is related to a practical fact, namely that although in theory the displacement can be set to zero, in practice it is very difficult to achieve this result with sufficient precision; in other words, there is no guarantee that a motor brought to the zero-displacement condition is really idle and uses no power or has such low inertia as to allow easy gear shifting.